Bacillus thuringiensis (Bt) gamma-endotoxins are widely used as insecticidal toxin against lepidoptera, diptera and coleoptera. Site-directed mutagenesis has been successfully used to improve protein toxicity against manduca sexta and gypsy moth. SDM combined with Biophysical techniques has provided a lot of information about the structure and function of the toxins. However, the membrane-bound state of the protein is still unknown. By the use of site-directed spin labeling, we expect to elucidate the structure of the protein in the membrane, and clarify which amino acids are required for protein insertion and ion channel activity. These findings, will help to design more toxic and more stable proteins by targeting the amino acids specific for insertion as shown in voltage clamp experiments and toxicity bioassays.